`mixed_language_library` is not usable on Linux because it depends on `objc_library`

[iXerol]

mixed_language_library currently expands to internal objc_library targets.

That makes non-Apple builds fail at analysis time due to objc_library requiring Apple toolchain behavior, which is expected in rules_cc:
https://github.com/bazelbuild/rules_cc/blob/ee57062efc3ab22132e28af782c70b6d73b052f5/cc/private/rules_impl/objc_semantics.bzl#L32

The issue is not with objc_library itself.
The issue is that mixed Swift + Clang-language interop is also a practical non-Apple use case, but mixed_language_library currently has no non-Apple path.

Would you consider:

1. supporting a non-Apple backend for mixed_language_library (for example a cc_library-based path for Clang-language sources), or
2. explicitly documenting that mixed_language_library is Apple-only?

[keith]

++, this TODO is even about that

rules_swift/mixed_language/mixed_language_library.bzl

Lines 360 to 362 in 9d2505f

	# TODO: Should this support `cc_library`, and if so, how do we choose?
	# We can't look at the `clang_srcs` attribute because it might be a
	# `select`.

seems to me like if we can't come up with another heuristic we should just allow the user to pass a bool / rule arg to override it